DNA: DNA Replication (3.2)

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Questions and Answers

What is the primary function of DNA polymerase during replication?

To synthesize new DNA strands by adding nucleotides (correct)

To unwind the DNA double helix

To remove RNA primers from the lagging strand

To join Okazaki fragments

Which enzyme is responsible for relieving torsional strain ahead of the replication fork?

Helicase

Ligase

Primase

Topoisomerase (correct)

What occurs at the origin of replication in the process of origin licensing?

The pre-replication complex is assembled (correct)

DNA strands are unwound to form a replication fork

Topoisomerases cleave the DNA strands

RNA primers are synthesized to initiate replication

Why is the 3' hydroxyl group critical during DNA replication?

It serves as the site for the addition of new nucleotides Signup and view all the answers

What is a consequence of errors in DNA polymerase activity during replication?

Potential mutations in the newly synthesized DNA Signup and view all the answers

What role does helicase play in DNA replication?

Unwinds the DNA at the origin Signup and view all the answers

Why is replication described as a semi-conservative process?

Each daughter DNA molecule contains one original and one new strand Signup and view all the answers

What is the significance of the 3'-OH group during DNA replication?

It is required for the extension of the DNA strand by polymerase Signup and view all the answers

What structures are formed on the lagging strand during DNA replication?

Okazaki fragments Signup and view all the answers

During DNA replication, how does topoisomerase contribute to the process?

It relieves torsional strain ahead of the replication fork Signup and view all the answers

What is the primary reason that eukaryotic organisms require multiple origins of replication on their chromosomes?

To increase the speed of replication Signup and view all the answers

Which enzyme is responsible for joining the Okazaki fragments together?

DNA ligase Signup and view all the answers

What happens if there are errors made during DNA replication?

They can lead to mutations in the genetic material Signup and view all the answers

What is the primary role of DNA polymerase III in prokaryotes?

Perform the main elongation during DNA synthesis. Signup and view all the answers

Which component is crucial for the origin licensing stage of replication initiation?

Origin Recognition Complex (ORC). Signup and view all the answers

What distinguishes early replicating regions from late replicating regions in DNA?

Early regions are associated with active gene transcription. Signup and view all the answers

In which way does RNA primer removal occur during lagging strand synthesis?

The flap is formed and cleaved by Flap Endonuclease (FEN1). Signup and view all the answers

What is the role of topoisomerases during DNA replication?

Unwind the DNA helix to relieve torsional strain. Signup and view all the answers

How does DNA polymerase δ primarily function during replication?

As the main polymerase of the lagging strand. Signup and view all the answers

What is a consequence of the end-replication problem?

The loss of genetic information over time. Signup and view all the answers

What characterizes the synthesis direction of DNA strands?

Synthesis occurs only in the 5' to 3' direction. Signup and view all the answers

Which of the following is NOT a function of telomeres?

Facilitate gene transcription. Signup and view all the answers

What is the primary function of the sliding clamps in DNA polymerase?

To stabilize DNA during synthesis. Signup and view all the answers

What distinguishes Okazaki fragments from other segments of DNA during replication?

They are formed during the synthesis of the lagging strand. Signup and view all the answers

Which unit of DNA polymerases provides proofreading capability?

ε subunit. Signup and view all the answers

What is an effect of transcription on replication timing?

It can contribute to overall genomic instability. Signup and view all the answers

Study Notes

Basics of Replication

Replication is a semi-conservative process, where each new DNA molecule contains one original strand and one newly synthesized strand.
Replication is bidirectional, meaning it proceeds in both directions from the origin of replication.
The origin of replication is where the DNA helix is unwound, and replication forks are created.
DNA helicases unwind the DNA double helix.
DNA topoisomerases relieve the strain ahead of the replication fork.
DNA polymerase adds new nucleotides in the 5' to 3' direction.
The lagging strand is synthesized discontinuously in Okazaki fragments.
DNA ligase seals the gaps between Okazaki fragments.

Origin Licensing

Replication licensing ensures that each DNA molecule is replicated only once per cell cycle.
The origin recognition complex (ORC) is a protein complex that binds to specific sequences in the DNA, called origins of replication.
The pre-replication complex (Pre-RC) is formed when ORC binds to Cdc6, Cdt1, and MCM helicase.
MCM helicase is a DNA helicase that unwinds the DNA at the origin of replication.
Once the pre-RC is formed, the origin is considered "licensed" for replication.

Origin Firing

Origin firing is the process of initiating DNA replication by activating the licensed origins.
During S phase, the Pre-RC is converted into a pre-initiation complex (Pre-IC).
The Pre-IC contains additional proteins that are required for DNA replication, such as DNA polymerase and primase.

DNA Synthesis

DNA polymerase is the key enzyme that synthesizes new DNA strands.
New nucleotides are added to the 3' hydroxyl group of the previous nucleotide.
Leading strand synthesis proceeds continuously in the same direction as the replication fork.
Lagging strand synthesis proceeds discontinuously in fragments.
Okazaki fragments are formed by the action of DNA polymerase and primase.

Polymerase Activities

DNA pol I is responsible for removing RNA primers and filling in the gaps between Okazaki fragments. It has 5' to 3' and 3' to 5' exonuclease activities.
DNA pol III is the primary enzyme for elongation of the leading and lagging strands.
DNA pol III has a 3' to 5' exonuclease activity that acts as a proofreading mechanism.

Topoisomerase

Topoisomerases relieve the strain on the DNA helix ahead of the replication fork.
Topoisomerases 1 cut a single strand of DNA, allowing it to rotate around the uncut strand.
Topoisomerases 2 cuts both strands of DNA, allowing the DNA to pass through the break before resealing the break.

The End-Replication Problem

The end-replication problem arises because linear chromosomes cannot be fully replicated by DNA polymerase due to the need for a 3' OH group to start synthesis.
This loss of DNA at the ends of chromosomes can cause shortening of the chromosomes.
Telomeres are specialized structures at the ends of chromosomes that help protect them from shortening.
Telomerase, a reverse transcriptase enzyme, extends the telomere repeat sequence at the ends of chromosomes to prevent shortening.

Replication in Different DNA Types

Circular dsDNA
- Replicates in one or multiple origins.
- Replicates bidirectionally.
ssDNA
- Requires a primer to start replication.
- The primer can be provided by a host cell or a specific polymerase.
Linear dsDNA
- Replicated by DNA polymerase.
- The lagging strand is synthesized discontinuously.
- The end-replication problem prevents the full replication of the ends.

Consequences of Errors in Replication Machinery:

DNA Polymerase Errors:
- Mutations can occur if errors in DNA polymerase are not corrected by the proofreading mechanism.
- Mutations can alter gene expression and lead to diseases.
Topoisomerase Errors:
- Failure to properly unwind the DNA strand can lead to DNA breaks and replication stalling.
- Mutations in topoisomerase can lead to cancer and other diseases.
Primase Errors:
- Errors in primase can lead to incomplete Okazaki fragment synthesis.
- Errors in primase can also lead to premature termination of replication.

Consequences of Errors in Replication

Insertions or deletions and point mutations can cause changes in gene expression.
Chromosomal rearrangements can lead to changes in gene order and may affect expression of genes.
Loss of genetic information can be the result of errors in DNA replication that can lead to disease.
DNA damage can also occur due to environmental factors and trigger changes in DNA sequence.

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Description

This quiz covers the fundamental concepts of DNA replication, including the semi-conservative nature of the process and the role of various enzymes such as helicases, polymerases, and ligases. Additionally, it discusses the mechanisms of replication licensing and the formation of the origin recognition complex. Test your understanding of these crucial molecular biology topics!